Psychologists Rachel Herz and Estelle Campenni were just getting to know each other, swapping stories about their lives over coffee, when Campenni confided something unexpected: She was living proof, she said, of love at first smell. "I knew I would marry my husband the minute I smelled him," she told Herz. "I've always been into smell, but this was different; he really smelled good to me. His scent made me feel safe and at the same time turned on—and I'm talking about his real body smell, not cologne or soap. I'd never felt like that from a man's smell before. We've been married for eight years now and have three kids, and his smell is always very sexy to me."

Everyone knows what it's like to be powerfully affected by a partner's smell—witness men who bury their noses in their wives' hair and women who can't stop sniffing their boyfriends' T-shirts. And couples have long testified to the ways scent-based chemistry affects their relationships. "One of the most common things women tell marriage counselors is, 'I can't stand his smell,'" says Herz, the author of The Scent of Desire.Sexual attraction remains one of life's biggest mysteries. We might say we go for partners who are tall and thin, love to cook, or have a mania for exercise, but when push comes to shove, studies show, the people we actually end up with possess few of the traits we claim to want. Some researchers think scent could be the hidden cosmological constant in the sexual universe, the missing factor that explains who we end up with. It may even explain why we feel "chemistry"—or "sparks" or "electricity"—with one person and not with another.

Physical attraction itself may literally be based on smell. We discount the importance of scent-centric communication only because it operates on such a subtle level. "This is not something that jumps out at you, like smelling a good steak cooking on the grill," says Randy Thornhill, an evolutionary psychologist at the University of New Mexico. "But the scent capability is there, and it's not surprising to find smell capacity in the context of sexual behavior." As a result, we may find ourselves drawn to the counter attendant at the local drugstore, but have no idea why—or, conversely, find ourselves put off by potential dating partners even though they seem perfect on paper.

Though we may remain partially oblivious to scent signals we're sending and receiving, new research suggests that we not only come equipped to choose a romantic partner who smells good to us, but that this choice has profound biological implications. As we act out the complex rituals of courtship, many of them inscribed deep in our brain, scent-based cues help us zero in on optimal partners—the ones most likely to stay faithful to us and to create healthy children with us.

At first blush, the idea of scent-based attraction might seem hypothetical and ephemeral, but when we unknowingly interfere with the transmission of subtle olfactory messages operating below the level of conscious awareness, the results can be both concrete and devastating. When we disregard what our noses tell us, we can find ourselves mired in partnerships that breed sexual discontent, infertility, and even—in extreme cases—unhealthy offspring.

The Scent of Desire

When you're turned on by your partner's scent, taking a deep whiff of his chest or the back of her neck feels like taking a powerful drug—it's an instant flume ride to bliss, however momentary. Research has shown that we use scent-based signaling mechanisms to suss out compatibility. Claus Wedekind, a biologist at the University of Lausanne in Switzerland, created Exhibit A of this evidence by giving 44 men new T-shirts and instructing them to wear the shirts for two straight nights. To ensure that the sweat collecting on the shirts would remain "odor-neutral," he supplied the men with scent-free soap and aftershave.

After the men were allowed to change, 49 women sniffed the shirts and specified which odors they found most attractive. Far more often than chance would predict, the women preferred the smell of T-shirts worn by men who were immunologically dissimilar to them. The difference lay in the sequence of more than 100 immune system genes known as the MHC, or major histocompatibility complex. These genes code for proteins that help the immune system recognize pathogens. The smell of their favorite shirts also reminded the women of their past and current boyfriends, suggesting that MHC does indeed influence women's dating decisions in real life.

Women's preference for MHC-distinct mates makes perfect sense from a biological point of view. Ever since ancestral times, partners whose immune systems are different have produced offspring who are more disease-resistant. With more immune genes expressed, kids are buffered against a wider variety of pathogens and toxins.

But that doesn't mean women prefer men whose MHC genes are most different from theirs, as University of Chicago evolutionary biologist Martha McClintock found when she performed a T-shirt study similar to Wedekind's. Women are not attracted to the smell of men with whom they had no MHC genes in common. "This might be a case where you're protecting yourself against a mate who's too similar or too dissimilar, but there's a middle range where you're OK," McClintock says.

Women consistently outperform men in smell sensitivity tests, and they also make greater time and energy sacrifices on their children's behalf than men do—in addition to bearing offspring, they look after them most of the time. These factors may explain why women are more discriminating in sniffing out MHC compatibility.